When is set direction used in vfx graph?

I'd like to change the direction of the particle's movement a little bit by bit.
I use set angle at this time, but can i give this effect with set direction?

 

Generally, you 'set' initial values in the Initialize Block, and you modify values over time in the Update Block.

There are lots of ways to do what you're asking (Add/Multiply Position/Velocity/Force). I often just keep experimenting to find something that I like

If you only use Set/Overwrite, you will constantly be setting the same value, which won't be a visible change over time